I'm no creative accountant but I've had a glace at a few balance sheets and result reports. What is possible (at least in my part of Europe) is that an airline may put leased aircraft in the balance sheet as an asset whilst at the same time, the lease agreement would go as a debt. Meaning, the balance sheet would look a lot bigger than it would've if you had not chose to do so. Assuming that the asset=debt this means equity ratio would fall. However, if the asset would be valued higher than the lease agreement, or dept, the equity would increase. This would especially be true should the depreciation of the asset be lower than the amortization of the lease/debt. It would introduce a cash flow problem however, but it would be possible to add on further debt to sort the cash flow. I've seen this happen in airlines where the owners interests of having and airline is blinding to the actual airline's possibility to generate any profit.
As for Norwegian, owned aircraft are considered an asset whilst leased aircraft are not. The depreciation plan is quite interesting as the residual value seem quite high whilst the depreciation plan is longer than most. From the FY16 report the residual value of the aircraft is 7 bn NOK whilst acquisition value is less than 27 (the report doesn't give exact acquisition value of the aircraft as a/c parts and installations made on leased aircraft are all put together). The depreciation plan is 25 years.
Having a look at the Q2 report it can be noted that Norwegian signed a LOI (realized during September) to sell 11 B738 and than lease them back immediately. This was taken into account in the balance sheet and results for Q2. The value of these aircraft in the balance sheet before the LOI was signed was 2,475 MNOK. However, the LOI only generated 1,899 MNOK, resulting in a loss of 576 MNOK for the 11 aircraft. Most of these aircraft were delivered in 2010. Of course, in Q3 the cash flow will show a significant increase.
My conclusion being that aircraft assets in Norwegian might not be worth the amount they hold in the balance sheet, meaning the company's equity is in reality lower than what is shown in the books. The aircraft value does not correspond to market value. Seven years after delivery, market value of these aircraft seem to be only ~75% of book value in the balance sheet.
How airlines should depreciate aircraft seem to be a topic for discussion among experts. A company with a new fleet holding on to their aircraft will in reality not be affected if the depreciation plan is flaw short term, but long term the effects will of course catch up. In the case of Norwegian, a 576 MNOK loss was not a problem given that NOFI shares were added to the balance sheet (and results) at fair value at the same time, adding 2 bn NOK to the balance sheet, but obviously not as much cash.